A new study has demonstrated that male tilapia cichlids use urinary odours to advertise their dominance within hierarchies.
The study, carried out at the University of Algarve, was based on the Mozambique tilapia, Oreochromis mossambicus, a maternal mouth-brooding cichlid found in freshwaters along the coast of East Africa.
The researchers found that the male tilapia would use chemical signals, released through pheromones in their urine, to ward off subordinate males in hierarchal and territorial disputes.
Dominant male tilapia were capable of storing large volumes of urine within their bladder than subordinate males. During symmetrical aggression, the frequency of urine release by the males would increase.
This increased urine release would end when either one of the two male tilapia became submissive, or when the aggression between the two males rose to mouth-to-mouth fighting or jaw-locking.
As well as being able to hold greater volumes of urine within their bladder, the study also found that dominant males were able to produced urine that contained a higher concentration of potent odorants than that of the subordinate males.
Male Mozambique tilapia form dense aggregations during the breeding season. Each fish within the community defends a small territory, in which they dig a nest.
The researchers believe that it is the ability of the dominant males to win the battle of chemical signals, by being able to send out stronger signals in their urine over a longer period of time than subordinate males within the community, which allows for the maintenance of social stability.
The study involved the University of Algarve s Mozambique tilapia brood stock, which were held in groups of one male to three or four females in 200 litre aquariums.
The urine of the male fish was made visible using a blue dye.
During the study it was found that, when isolated, the male tilapia would urinate at a low rate of one pulse every 10 minutes.
This frequency of urination would increase ten-fold when another male was introduced into the tank.